linux-kernel - Re: [PATCH v1 2/3] blk-mq: comment on races related with timeout handler

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Date:   Fri, 17 Mar 2017 17:39:06 +0000
From:   Bart Van Assche <Bart.VanAssche@...disk.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "hch@...radead.org" <hch@...radead.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "tom.leiming@...il.com" <tom.leiming@...il.com>,
        "axboe@...com" <axboe@...com>
CC:     "yizhan@...hat.com" <yizhan@...hat.com>
Subject: Re: [PATCH v1 2/3] blk-mq: comment on races related with timeout
 handler

On Fri, 2017-03-17 at 17:57 +0800, Ming Lei wrote:
> +/*
> + * When we reach here because queue is busy, REQ_ATOM_COMPLETE
> + * flag isn't set yet, so there may be race with timeout hanlder,
> + * but given rq->deadline is just set in .queue_rq() under
> + * this sitation, the race won't be possible in reality because
> + * rq->timeout should be set as big enough to cover the window
> + * between blk_mq_start_request() called from .queue_rq() and
> + * clearing REQ_ATOM_STARTED here.
> + */
>  static void __blk_mq_requeue_request(struct request *rq)
>  {
>  	struct request_queue *q = rq->q;
> @@ -700,6 +709,19 @@ static void blk_mq_check_expired(struct blk_mq_hw_ctx *hctx,
>  	if (!test_bit(REQ_ATOM_STARTED, &rq->atomic_flags))
>  		return;
>  
> +	/*
> +	 * The rq being checked may have been freed and reallocated
> +	 * out already here, we avoid this race by checking rq->deadline
> +	 * and REQ_ATOM_COMPLETE flag together:
> +	 *
> +	 * - if rq->deadline is observed as new value because of
> +	 *   reusing, the rq won't be timed out because of timing.
> +	 * - if rq->deadline is observed as previous value,
> +	 *   REQ_ATOM_COMPLETE flag won't be cleared in reuse path
> +	 *   because we put a barrier between setting rq->deadline
> +	 *   and clearing the flag in blk_mq_start_request(), so
> +	 *   this rq won't be timed out too.
> +	 */
>  	if (time_after_eq(jiffies, rq->deadline)) {
>  		if (!blk_mark_rq_complete(rq))
>  			blk_mq_rq_timed_out(rq, reserved);

Since this explanation applies to the same race addressed by patch 1/3,
please consider squashing this patch into patch 1/3.

Thanks,

Bart.